ABSTRACT

In a channel expansion, flow can separate from the bank, creating a zone of relatively weak recirculating current. Bars that accumulate in this weak flow near the point where flow reattaches to the bank are called reattachment bars. As a reattachment bar evolves, the recirculation zone may fill with sediment and restrict flow from the main channel. The increasingly restricted flow over the bar causes ripples to replace dunes and causes the sediment size to fine; the resulting vertical sequence resembles that of point bars. Seasonal and daily flow fluctuations in the Grand Canyon complicate this idealized sequence. Changes in discharge alter the geometry of recirculation zones, flow within the recirculation zones, the location of depositional and erosional sites, the kind of bedform an migration direction of bedforms on the bar, and the transported sediment size.

Dunes and ripples within a recirculation zone migrate in a rotary pattern in response to the recirculating flow. Ripples near the reattachment point often resemble oscillation ripples in morphology and dynamics. The reversing flow that creates these ripples is caused by fluctuations in location of the reattachment point. These fluctuations cause flow near the reattachment point to reverse in an upstream-downstream direction, thereby producing symmetrical, reversing ripples with crests that trend normal to the bank. Low rates of ripple migration in the reversing flow, accompanied by rapid deposition, cause these ripples to climb at a high angle. At increasing distances from the reattachment point, the reversing flow is less balanced, and the ripples climb at lower angles as they migrat upstream and downstream.

Although these observations were made in a bedrock canyon, the same processes operate in alluvial and tidal channels and are important in adjusting the shape of channels on point bars and concave benches and behind bedforms that become emergent at low stage. Reattachment bars can be recognized by the rotary flow patterns and by symmetrical, reversing, vertically climbing ripples.